I work as a trainer in the gym and have done hundreds of assessments and have tons of clients month to month and can honestly say, I've never needed to know the exact amount of BF% a person has. Most BIA devices or calipers may be off by a few % points, but they will always be accurate when assessing the amounts lost or gained since it's just the generic formula it uses to do the calculations that may be wrong depending on your distribution of fat in certain sites but will always do the same calculation and you will always carry your fat distributed in the same way.